Origin of corona-dominated topographic rises on Venus

Both large-scale mantle upwellings, comparable to terrestrial hotspots on E arth, and smaller scale mantle upwellings, known as coronae, occur on Venus . Corona-dominated rises have many of the characteristics of large-scale ma ntle upwellings, or hotspots, such as broad topographic, rises greater than 1000 km in diameter and large positive gravity anomalies. Due to the prese nce of clusters of three to eight coronae, three large volcanic rises (or h otspots) on Venus have been classified as corona-dominated rises (CDRs): Th emis, Eastern Eistla, and Central Eistla Regiones. CDRs have been interpret ed to result from the break-up of a large-scale plume. Comparison of the to pographic morphology for individual coronae at Themis and Eastern Eistla Re gion to a model of corona evolution indicate that they are in varying stage s of evolution. At Eastern Eistla Regio all the coronae have essentially th e same topographic morphology, consistent with a late stage of evolution an d the presence of a depleted mantle layer at depth. The complex deformation sequences and stratigraphic relationships both between coronae and with re spect to the regional plains observed at all three rises indicate a prolong ed origin.:This observation, as well as the varying stages of evolution, ru le out the previously proposed interpretation of corona-dominated rises as a manifestation of the break-up of a large-scale mantle upwelling, which re quires essentially simultaneous formation of the coronae. Instead we sugges t that other large topographic rises are the manifestation of deep mantle p lumes, likely to originate at the core-mantle boundary, and that CDRs are c lusters of coronae that originate at a shallower interface, perhaps at an u pper-lower mantle boundary. Using top- and bottom-loading flexural models t o fit the gravity/topography admittance spectrum for each of the three CDRs yields elastic thickness estimates that are 10-15 km greater for bottom-lo ading at longer wavelengths than top-loading at shorter wavelengths. Estima tes of elastic thickness assuming top-loading are 10, 12, and 22 km and 20, 25, and 35 km from bottom-loading for Eastern Eistla, Central Eistla, and Themis Regiones, respectively. Wt: believe that the bottom-loading elastic thickness estimates are more reliable because using a top-loading model whe n both types of loading are present yields an unrealistically low elastic t hickness estimate. As there is no obvious source of surface loading at eith er Themis or at Eastern Eistla, we interpret the top-loading admittance sig nature to be a result of delamination of the lower lithosphere depressing t he surface, which is consistent with the observed coronae morphologies. (C) 1999 Academic Press.